Breadth of glass transition temperature in styrene/acrylic acid block, random, and gradient copolymers: Unusual sequence distribution effects

Block, random, and gradient copolymers of styrene (S) and acrylic acid (AA) are synthesized by conventional or controlled radical polymerization, and their glass transition temperature (T-g) behaviors are compared. The location and breadth of the T(g)s are determined using derivatives of differential scanning calorimetry heating curves. Each S/AA random copolymer exhibits one narrow Tg, consistent with a single phase of limited compositional nanoheterogeneity. Block copolymers exhibit two narrow T(g)s originating from nanophase separation into ordered domains with nearly pure S or nearly pure AA repeat units. Each gradient copolymer exhibits a T-g response with a similar to 50-56 degrees C breadth that extends beyond the upper Tg of the block copolymers. For copolymers of similar composition, the maximum value in the gradient copolymer Tg response is consistent with that of a random copolymer, which has an enhanced T, relative to poly(acrylic acid) due to more effective hydrogen bonding when AA units are separated along the chain backbone by S units. These results indicate that gradient copolymers with ordered nanostructures can be rationally designed, which exhibit broad glass transitions that extend to higher temperature than the T(g)s observed with block copolymers. (c) 2007 Wiley Periodicals, Inc.